In conventional redox flow battery systems, a cation exchange membrane is generally used. When a cation exchange membrane is used during the operation of a redox flow battery, a phenomenon is observed in which vanadium is transferred from the negative electrolyte to the positive electrolyte during redox reactions. To reduce the change in vanadium concentration caused by this transfer, a large amount of water is also transferred from the negative electrolyte to the positive electrolyte.
It was reported that, when a battery was constructed using Nafion 115, which is a general cation-exchange membrane, and positive and negative electrolytes having the same initial volume, about 7-10% of the electrolyte was transferred to the positive electrode of the battery (Investigations on transfer of water and vanadium ions across Nafion membrane in an operating vanadium redox flow battery.
The difference in volume between the negative electrolyte and the positive electrolyte increases as discharge is repeated, and this phenomenon causes a problem in that a free space should be ensured in electrolyte storage tanks. This problem is more serious when the size of the battery is increased for commercial use. Ensuring the free space based on the expectation of a change in the volume of the electrolytes is disadvantageous in economic terms, and can also adversely affect the durability of the storage tank by changing the internal pressure of the storage tank.
In addition, if a large amount of water is transferred to the positive electrode, the concentration of anions will decrease in the positive electrolyte and increase in the negative electrolyte to thereby reduce the stability of the electrolytes, because anions are not freely transferred due to the cation exchange membrane. For example, it is already well known that, if sulfate ions are used as a supporting electrolyte, V2+ and V3+ ions in the negative electrolyte are unstable at high sulfate ion concentrations to cause problems such as the precipitation of vanadium.
In the prior art, an attempt was made to solve the problem in that the state of the electrolytes changes after charge and discharge, by restoring transferred ions or controlling the diffusion coefficient of ions.
As a method for restoring transferred vanadium ions, studies focused on restoring vanadium ions to the initial state by remixing a positive electrolyte and a negative electrolyte after use have been conducted. Representative examples of this method include a total mixing method in which the two electrolytes are mixed and divided into two portions, and a partial transfer method in which a portion of a positive electrolyte, determined on the calculated amount of transferred vanadium ions, is transferred to a negative electrolyte.
However, the total mixing method and the partial transfer method all have disadvantages in that it is difficult to accurately calculate the amount of vanadium ions and in that the risk of heat generation cannot be avoided when the two electrolytes are mixed with each other in a state in which they are not completely discharged. An operation of completely discharging two electrolytes during the use of the battery in real life is troublesome and inconvenient, and the time required to mix the two electrolytes with each other increases as the size of the battery increases. For this reason, it is actually impossible to frequently perform the total mixing method. In addition, even when the electrolytes are mixed by the partial transfer method, they are not restored to a state completely identical to the initial state, and this problem is also to be solved. Although vanadium ions transferred to the positive electrode can be restored, anions are highly likely to accumulate in the negative electrolyte, because anions are also transferred during the transfer of vanadium ions.
Other prior technologies include a method of reducing the transfer of specific ions using an improved exchange membrane. However, because a mechanism by which ions passes through the exchange membrane is not perfectly known, it is difficult to study this technology, and this technology is expected to be put to practical use and commercialized after a considerable amount of time.